Case 68: Pneumatic testing dangers

Ive written previously on
hydro and pneumatic testing and how its an
engineers responsibility to question dangerous
conditions.1,2 Unfortunately, I still see failures
and have been asked what is a safe distance to be at when
performing a pneumatic test.3,4

When defining safe
distances from a pneumatic explosion, a major concern is
defining the numerous flying fragments. To understand the
energy involved in pneumatic testing, the pressurized air will
be treated as a compressed spring. The energy released will be
used to propel a fragment horizontally. Fig. 1 shows a
fictitious gas spring in a vessel with a spring constant,
k, in lb/in., and compressed d, in. The diameter of
the fragment is D, and the pressure is p,
lb/in.2 In this example, d could be the length of a
pipe or the diameter of a vessel, d. The pneumatic
spring constant is:

k = p 3 (π/4)
D2 / d (lb/in.)

The potential energy (PE) in a
spring is:

PE = ½ k X
d2 = ½ p X (π/4)
D2 X d in.-lb

The kinetic energy (KE) of the
fragment W is:

KE = ½ (W/g
) X V2

Equate KE = PE and solve for
fragment velocity, V:

V = 17.4D
(p X / W
)1⁄2 in./sec

Fig. 1. A compressed air
model.

This velocity can now be used in a
simple trajectory calculation to estimate the distance that the
fragment will travel horizontally. R is the horizontal
range that W travels at initial velocity, V,
from a height, h, ft:

R = (V/24) X
( 2 X h/32.2
)1⁄2 ft

Fig. 2 represents the
fragments flight path with averaged velocity. Fragments
of different sizes will have varying ranges. Stating a safe
distance isnt possible unless you know the size. For
example, with a 200-lb/ in.2 pneumatic explosion of
a pipe with D = 10 in., h = 10 ft and d = 50
in, a fragment, W = 0.1 lb, will have R =
1,800 ft. However, with a fragment of W = 25 lb and
R = 114 ft, there are many possibilities.

Fig. 2. Fragment range and flight
path of debris.

Detailed trajectory calculations considering departure angles
and air friction still require that these variables are
defined. Its always prudent to look for alternatives when
pneumatic testing large volumes.

Options such as localized
hydrotesting with small volumes involve sections of pipe
instead of the whole pipeline. Nondestructive testing of welds
and critical areas is another option. Consider reviewing the
historical hydrotesting of a modified vessel and only testing
the portions that were modified can be used. This may require
temporary weld on caps. Strive to recognize codes if you must
pneumatically test. Remember: There may be no
reasonable safe distance or exclusion
zone. The safe distance specified could reference a blast
pressure wave, not flying fragments.

A ruptured 25-lb pipe-end fragment
with an air volume of 140 ft3 at 200
lb/in.2 is like a locomotive engine traveling at 15
mph or one pound of TNT. All possess about 1.5 million ft-lb of
KE. It is something to seriously consider when discussing
pneumatic testing. HP

Dr. Tony Sofronas, P.E., was the
worldwide lead mechanical engineer for ExxonMobil
before his retirement. He is now the owner of
Engineered Products, which provides consulting and
engineering seminars. He can be reached through the
website http://mechanicalengineeringhelp.com by
clicking on the comments/question tab.

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Thank you for your comments. The equation was correct in the magazine but in error here. rho 3 should be p and d should be delta in some of the equations. Must have been a problem going electronic. 24 is correct as 1/12*1/2.

vincenzo puccia02.22.2014

probably two printing error: the 3 number in the pneumatic spring constant and the 24 under V (1 ft=12 in, not 24)

Thank YouVP

franco balestri09.04.2013

In the final formula for R, velocity V should be divided by 12 instead of 24?